Four prison officers at Hydebank Wood have tested positive for Covid-19.
Two prisoners have been put into isolation "as a precaution", a spokesperson for the NI Prison Service said.
A further 14 members of staff, who have tested negative, are self isolating.
Last week, a prisoner tested positive for coronavirus at Maghaberry jail and on Friday all visits to prisons in Northern Ireland were suspended for four weeks.
Hydebank Wood houses young male offenders at its secure college and female prisoners in Ash House.
"The Prison Service has well-established and embedded arrangements for testing staff, and to track and trace close contacts following a positive case," said the spokesperson.
"The safety of staff and the people in our care is our priority and the Prison Service will take all necessary steps to mitigate against the risk of Covid-19 in our prisons."